1. Field of the Invention
The invention relates to a cross-linked epoxy resin and method for producing the same; particularly, the epoxy resin of the invention with flame-retardant properties has better thermal stability and mechanical property.
2. Description of the Prior Art
Epoxy resin is widely used in many domains such as paint, adhesive, building materials, electric packing materials, etc. Because it has many advantages such as no small molecules forming during curing process, low shrinkage rate for finished products, low strain, resistant to acid, alkali and other chemical solutions, good adherence, good insulation, light weight, low cost, easy to treat, etc. In recent years, epoxy resin even gradually replaces traditional materials to be used in high technology regions such as electric information, mechanical engineering, aerospace engineering, etc.
However, poor mechanical property and inflammability of epoxy resin often induce unexpected defect on products of top technology regions such as electronics, composite materials, etc. Therefore, how to improve the mechanical property and inflammability of epoxy resin becomes a popular research direction.
There are mainly three common methods for modifying the epoxy resin:
(1) Introducing non-reaction type of long chain molecules, for example, natural or synthetic rubber, and so on.
(2) Introducing reaction type of long chain molecules. For example, imide oligomer, aromatic polyester, etc.
(3) Increasing the cross-linked density or the intramolecular interactions.
It is effective to enhance the flexibility of epoxy resin by adding rubber or thermoplastic particle. However, this method also decreases its glass transition temperature, yield stress and heat stability at the same time. In recent years, many researches have shown that they can effectively raises the flame-retardant and heat-resistant properties of polymeric materials by introducing a phosphorus-containing flame-retardant agent and replacing the halogens by other elements. Since the phosphorus-containing flame-retardant agent has many advantages such as hypotoxicity, low smoking generation and low recruitment, there is a tendency toward developing the phosphorus-containing flame-retardant agent in recent years.
In general, there are two ways to get a flame-retardant epoxy resin:
One is adding a flame-retardant agent to an epoxy resin. The other is directly introduction of flame-retardant elements into the molecular structures of an epoxy resin or a curing agent.
However, there are some disadvantages to raise the effect of flame-retardant by adding a flame-retardant agent. First, it will induce poor compatibility between the flame-retardant agent and the epoxy resin. Second, it will influence the workability of the epoxy resin. Third, the flame-retardant agent easily migrates to the surface of an object. However, the reaction type flame-retardant agent has advantages of preventing migration, less contamination, better durability, water resistance, etc., so that the reaction type flame-retardant agent has become a research focal point in recent years.
In 1996, M. D. Shau et al. synthesized a phosphoric tri-amine compound (TAPO) to be the epoxy resin curing agent to carry on the research of the flame-retardant nature. In 1999, M. D. Shau used the phosphoric amines curing agent (BAMPO) to prepare the phosphoric epoxy resin, and then it achieved the effect of flame-retardant. In 1999, C. S. Wang et al. took DOPO phosphoric group as main bodies, developed a series of a phosphoric group curing agent, a phosphoric epoxy resin and a phosphoric epoxy resin half solidification which have high glass transition temperature, high heat-resistant property and extremely good flame-retardant property. It is important to note that 9,10-Dihydro-9-oxa-10-phosphaphenthrene (DOPO) is a kind of phosphoric organic compound, developed in 1970, and its reactive hydrogen is easy to react with electrophilic compounds such as rosolic acid, benzoquinone, bismaleimide, oxirane, diaminobenzophenone, maleic acid, cyanate ester terephthaldicarboxaldehyde, and so on. DOPO can be used to be the forerunner of a flame-retardant additive or a reaction type flame-retardant agent.